Girders, such as beam girders, I-beams, and box girders, are commonly used for construction of various structures such as bridges, roofs, and floors. A beam girder is an efficient system which transfers shear and load between the extreme upper and lower elements of the beam. However, for beam girder structures designed for a uniformly applied load per foot of bridge span, the bending moment increases by the square of the length of the structure. This rapid increase in the bending moment as a function of the structure length is disadvantageous because one way to counter the bending moment is by increasing the girder beam size, and this results in large increases in girder beam size with relatively small increase in the length of the structure. Thus, it is desirable to find a way to significantly reduce the required size of a beam girder in proportion to its length.
U.S. Pat. No. 6,493,895 to Reynolds (the '895 patent) provides a truss segment positioned on the upper side of a bridge girder. The truss segment is centered near the lengthwise midpoint of the girder and acts to counter the bending moment. The truss segment has the shape of an M with a bar across the top. The truss segment experiences strain and develops a load in its framework when the primary bridge girder bends and deflects under the load. When the primary bridge girder is subjected to a vertically downward load, the compact truss mechanism experiences compression on the vertical elements of the M, tension on the diagonal elements of the M and compression on the horizontal bar across the top of the M. In a primary bridge girder subjected to a uniformly applied vertical load, the combination of forces developed by the truss segment exerts a force couple upon the girder, within the extent of the truss mechanism, that is contrary to the bending moment of a conventionally girder. In this way, the bending and deflection of the primary bridge girder causes the boundary elements of the truss segment to oppose the bending and deflection of the primary bridge girder. This opposition reverses the bending moment at the girder's midpoint preventing the maximum bending moment of the girder from occurring at the midpoint, as in a conventional girder.
The purpose of the truss segment is not to create a geometrically rigid triangle to support bridge loads, as in a conventionally designed truss, but to alter the deflection curvature of the beam and to redistribute the bending moment through a lever type action upon the beam girder. Long diagonal segments placed at the top joints of the truss framework and extending to the end points of the girder span are placed in compression when the bridge is subjected to a load exerting a vertically downward force. This occurs in the long diagonal segments because of the vertically downward deflection of the primary bridge girder and the opposition to rotation that the truss segment itself creates in compression across its top boundary.
The effect of the action of the structural truss mechanism and the long diagonal segments that interact with the structural mechanism is to diminish the maximum bending moment of the girder, to relocate the maximum bending moment nearer to the end support point of the girder and to significantly reduce girder deflection. A truss segment, placed within the span of a girder, can be used to reduce girder stress and deflection at midspan when it is actuated by the movement and angular deflection of the primary girder. In other words, the opposing force that acts upon the primary girder by the truss mechanism is triggered by the deflection of the primary girder to which the truss segment is attached.
Although the truss segment described in the '895 patent provides a solution to the problem of countering the bending moment of a long girder without having to increase the girder size dramatically, it has its disadvantages. For example, because the truss segment is placed on top of the girder, it takes up extra space by adding depth or dimension to the total girder system. The extra space taken up above the midpoint of the girder poses an inconvenient design limitation. Furthermore, where the position of the enhancement transmits loads as shear into the girder, acting at the neutral axis rather than reacting against the girder, the girder accommodates the shear as a stress imposed upon the girder. Also, the system in the '895 patent does not provide for a differentiation of materials between the girder and its coupled enhancement as well as a standardization of connections and assembly.
A girder system that counters the bending moment like the truss segment of the '895 patent but does not suffer from the above shortcomings is desired. Also desired are: 1) a method of removing shear, initially expressed as axial compression in the enhancement structure, from being transmitted to the girder, 2) a method of merging the enhancement with the coupled girder so that different materials may be combined to achieve optimum result, and 3) a method by which connections may be standardized to allow the desirable features of compact shape, minimal transmission of shear, and easy conformance of different materials.